Field of the Invention
The subject invention relates to a musical apparatus and more particularly to circuitry for detecting which note or notes have been selected by a musician on a guitar or other fret board. Such circuitry finds particular use in providing control signals to an electronic synthesizer.
Numerous attempts have been made to develop a universal guitar-to-synthesizer interface. Since synthesizers themselves are now quite advanced, much of this effort has been in the area of the guitar itself. The prime problem has been in accurately determining the notes that a guitarist is playing in order to direct the synthesizer to play the corresponding notes. Two general methods have produced somewhat fruitful results: positional sensing and time extraction. Positional sensing methods usually involve utilizing the metal strings and metal frets in a switching matrix to determine where a particular string is depressed against a fret. To facilitate polyphonic note determination (more than one note at a time), this method has required each of the metallic frets to be split into six insulated segments - one for each string at each fret (see U.S. Pat. No. 3,482,029). This is a costly, mechanically deficient method but variations of this method have seen some commercial usage.
The time extraction method is typically a period measurement technique where the actual vibrational output of the strings is filtered and processed to yield a voltage corresponding to the note being plucked. This method is susceptable to a variety of problems including string-to-string interactions noticeable delays in note determination, and various noise-induced phenomena. However, this has been the most commercially successful method since it allows normal user controlled musical nuances to be applied such as string bending, hammers, slides, etc. Reliability in tracking the individual notes has been the severest problem of this method and has probably done more to cause user resistance to guitar control of synthesizers than any other single reason.
A typical guitar comprises six metal strings stretched across a neck and a companion body. These strings may vary in diameter from 0.009 to 0.043 inches (from the highest frequency string to the bass string). Normally, these strings are electrically described as being pure conductors which implies that they have zero resistance. However, with proper instrumentation, it can be shown via measurements, that the resistance of such strings is not truly zero. In fact, engineering data books tabulate resistances of various types of metallic wires as standard reference data. Data from such sources indicates that the resistance of steel wire of the diameters used on a guitar would be only a few ten thousandths of an ohm over the full length of a typical string.
It has occurred to the inventor that it would be advantageous to somehow utilize this resistive property of the strings to allow determination of positional information pertaining to where the string is depressed against a metal fret. It might appear possible theoretically to measure the resistance of a string from the bridge to the point that it touches a particular fret. Knowing the resistance-per-inch of that string would then allow detection of the length between the bridge and the fret and thus the note depressed. However, a number of practical considerations make this method unusable. First of all, the resistance of the string is so small that the resistance of the fret-to-string contact becomes significant in comparision. Also, as the strings age and become dirty and stretched, the resistance varies in an unpredictable way. Also, since none of the strings are the same diameter, even changing strings can cause all the circuitry to require readjustment. As more than one string is depressed, the measurements on a particular string become even more unpredictable due to the paralleling of the strings and resulting dropping of effective resistance.
Thus, any method using string resistance to determine positional information should be independent of string size, string aging, number of strings depressed, topology of the fingering on the neck, etc. It is an object of the invention to provide an apparatus that satisfies all the above requirements by using a "go/no-go" method of resistance measurements. Another object of this invention is to allow accurate polyphonic reproduction of guitar notings utilizing the reliability of positional sensing without resorting to costly, unreliable modifications to the guitar, which are required by prior art positional sensing methods. It is a further object of this invention to allow the guitarist maximum artistic control of the frequency of his notings.